Beefing up the floor slabs at the central core of the Cosmopolitan Tower will allow the units to have open floor plans. Click here for a larger image.

A new method of designing residential towers' structural systems is resulting in longer floor spans and more usable living space.

Used in two downtown Seattle condominium projects under construction  the 34-story Cosmopolitan Tower at Ninth and Virginia and the 22-story Madison Tower at First and Madison  the technique eliminates a ring of internal columns that typically shorten the floor spans and limit space planning options.

Contrived by Bellevue structural engineer Cary Kopczynski and Co., the method involves thickening up the concrete floor slabs in the corridors surrounding the core where floor-to-ceiling height is less critical. The design improves the efficiency of the core and allows for post-tensioned floor spans to continue from the core to the exterior of the living units without additional internal support.

The character of these slabs differs greatly from that of conventional post-tensioned flat slabs, which typically have a span capability of around 30 feet. With most core-to-exterior-wall floor spans ranging between 35 and 40 feet, residential tower designs usually dictate a row of support columns between 5 and 10 feet from the core wall. Using beams in place of columns isn't a much better solution: Since a post-tensioned slab doubles as the ceiling of the unit below it, there's no space to hide horizontal supports.

How it works

The system works by placing more concrete in the corridor ceiling space where electrical wiring and sprinkler systems are typically housed. Ductwork is redirected from the corridors to the dropped ceilings in the units' foyers, and the slab thickness is increased from a typical 8 inches to as much as 18 inches for the width of the corridors. The additional thickness around the building's center puts more of its vertical load on the core, which, in turn, supports the longer floor spans.

Images courtesy of Cary Kopczynski and Co.

The Madison Tower, under construction in Seattle’s West Edge neighborhood, was designed without the typical ring of interior columns that block views and limit layout.

Eliminating the internal columns provides a number of benefits; the additional flexibility gained inside the units being the most obvious. Architects are free to design the living units in a number of ways without structural columns limiting floor plans. The tenant benefits as well, since columns aren't blocking views or getting in the way of furniture, appliances and decor.

"Being able to remove that row of columns makes all the difference in the world," said Cary Kopczynski, president of Cary Kopczynski and Co.

Fewer columns also mean fewer space allocation challenges in the building's lower levels. Eliminating the interior columns removes the need to run them down through the lobby and parking areas. In a building with a relatively small footprint, numerous columns in a lower level or below-grade garage can create circulation and space planning issues.

Thicker floor slabs surrounding the core also result in a more structurally efficient building. Depending on the building size, the technique places more than 40 percent of the total weight on the core. By transferring the load from the columns to the core, the shear walls become more efficient in resisting seismic and lateral forces. It also cuts reinforcing steel requirements in the core by 10 to 20 percent; a bonus, as steel prices have nearly doubled in the past few years.

Already in use

Cary Kopczynski and Co. first used this design on the Cosmopolitan Tower in 2000, working closely with Seattle architect Mithun and the developer, Continental Properties. The project was put on hold, but general contractor Mortenson of Bellevue recently resumed construction. A completion date has been set for early 2007.

The method was also used for Madison Tower, a hotel/condominium tower that topped out in October. Here, the thicker slabs in the corridors are used on two sides of the core. Designed by Seattle's Weber + Thompson and built by Bellevue general contractor/developer 1000 First Avenue Properties LLC, the tower is expected to be complete in mid-2006.

The method of increasing residential tower floor spans is most effective on buildings designed with flat-slab construction and corridors wrapping around the core. However, even projects that aren't originally conceived to follow this layout can be altered to fit the bill. Close communication between the structural engineer, architect and owner at the beginning of the design process can result in minor design changes that move off-set corridors to the core, allowing for use of the column-eliminating design.

"It's really the function of the engineer to bring the possibility of this type of design to the table early enough so implementation is possible," said Kopczynski. "Minor shifts made before drawings are well under way can mean the difference between living units with restrictive columns and those with floor plans that offer enhanced possibilities."

Sheila Bacon is a freelance writer covering the Pacific Northwest's architecture, engineering and construction communities. She can be reached at sheila@sheilabacon.com.